Quantum dots have generated an increasing interest because of their narrow and size-tunable emission spectra, broad absorption profiles, high quantum yields, and good chemical stability. One of the most promising applications of these luminescent quantum dots is served as an alternative fluorophore for molecular, cellular, and in vivo imaging. They have the potential to overcome many of the limitations encountered by conventional organic fluorophores and genetically engineered fluorescent proteins in a variety of biological applications. For biomedical applications, it is desirable to preserve the water dispersible and highly luminescent properties of the quantum dots. However, high-quality luminescent quantum dots are usually synthesized in organic media, and the resulting quantum dots capped with the native hydrophobic ligands (e.g., trioctylphosphine oxide, alkyl amines etc.) are not soluble in aqueous solutions.